We propose a 3-year, qualitative exploratory study of 'late initiates' {individuals who began injecting drugs at age 30 or later) and 'typical initiates' {those who initiate injection prior to 30) in order to better understand the circumstances, motivations, social environments and HIV, HCV, and HBV risk associated with injection initiation later in life. There are an estimated 3 million injection drug users {IDUs) in the US. Many of the detrimental health consequences of illicit drug use, including HIV, HCV, and HBV, as well as soft-tissue infections, and overdose, are related to the act of injection. Between 15% and 19% of IDUs initiated drug injection after turning 30 years old. Little is known about how late initiates differ from typical initiates {initiating before age 30). We propose to examine how aspects of drugs, individual characteristics, and social environments affect transition to drug injection among adults 30 years of age and older. While there are more than a dozen published papers on initiation of drug injection, most studies have focused on young injectors and the specific circumstances of the first injection episode, and do not systematically consider how route of administration might intersect with drug use trajectories, life events, and accumulated health, social, and psychosocial vulnerabilities {e.g., HIV/AIDS and other conditions, multiple incarcerations, traumatic events, intergenerational poverty). This gap in knowledge is significant since late initiates are likely to need different approaches to HIV and substance abuse prevention and treatment than adolescents and young adults. The primary goals of the proposed qualitative study are to understand and compare late initiates to typical initiates and to provide a thick description of the intersections amongst drug use, life events, HIV risk and infection, social and physical environments, and initiation of drug injection. Our proposal uses Zinberg's classic framework of Drug, Set, and Setting {DSS) and lifcourse theory {LCT) to guide the aims, research questions, and analyses of this exploratory study. We have established the following aims. Aim 1: To explore attributes of drugs and drug injection (DRUG) among late initiates using LCT constructs. Aim 2: To describe individual characteristics, including HIV status and perceptions of HIV/AIDS risks, drug use histories, and key life events (SET) of late initiates as compared to typical initiates using LCT constructs. Aim 3: To explore the interpersonal, social, and structural environments (including the prevalence of HIV/AIDS and other infectious diseases in the community- SETTING) of late initiates compared to typical initiates using LCT constructs. To accomplish these aims, we propose to conduct a qualitative, exploratory study comparing 80 late initiates and 80 typical initiates. Participants will be HIV negative and HIV positive, active IDUs recruited from community settings using targeted sampling methods. We will use the grounded theory method and life event approaches to explore in-depth the concepts related to DRUG, SET, and SETTING that influence late initiation to injection drug use. To capture a diversity of IDUs and settings, we will recruit IDUs in Los Angeles and San Francisco, two cities that have important differences in IOU demographics, drug use preferences, the incidence and prevalence of HIV/AIDS among IDUs, and other community and environmental influences. Exploring DSS among late initiates will help to identify influential factors for the prevention of injection drug use, injection-related risks for HIV/AIDS, and other health and social consequences.